Velocity data from a mooring array deployed northeast of the Campeche Bank (CB) show the presence of subinertial, high-frequency (below 15 days) velocity fluctuations within the core of the northward flowing Loop Current. These fluctuations are associated with the presence of surface-intensified Loop Current frontal eddies (LCFEs), with cyclonic vorticity and diameter < 100 km. These eddies are well reproduced by a high-resolution numerical simulation of the Gulf of Mexico, and the model analysis suggests that they originate along and north of the CB, their main energy source being the mixed baroclinic–barotropic instability of the northward flow along the shelf break. There is no indication that these high-frequency LCFEs contribute to the LC eddy detachment in contrast to the low-frequency LCFEs (periods > 30 days) that have been linked to Caribbean eddies and the LC separation process. Model results show that wind variability associated with winter cold surges are responsible for the emergence of high-frequency LCFEs in a narrow band of periods (6–10 day) in the region of the CB. The dynamical link between the formation of these LCFEs and the wind variability is not direct: (i) the large-scale wind perturbations generate sea level anomalies on the CB as well as first baroclinic mode, coastally trapped waves in the western Gulf of Mexico; (ii) these waves propagate cyclonically along the coast; and (iii) the interaction of these anomalies with the Loop Current triggers cyclonic vorticity perturbations that grow in intensity as they propagate downstream and develop into cyclonic eddies when they flow north of the Yucatan shelf.

We carry out assessments of the life cycle of Loop Current vortices, so-called rings, in the Gulf of Mexico by applying three objective (i.e., observer-independent) coherent Lagrangian vortex detection methods on velocities derived from satellite altimetry measurements of sea-surface height (SSH). The methods reveal material vortices with boundaries that withstand stretching or diffusion, or whose fluid elements rotate evenly. This involved a technology advance that enables framing vortex genesis and apocalypse robustly and with precision. We find that the stretching-and diffusion-withstanding assessments produce consistent results, which show large discrepancies with Eulerian assessments that identify vortices with regions instantaneously filled with streamlines of the SSH field. The even-rotation assessment, which is vorticity-based, is found to be quite unstable, suggesting life expectancies much shorter than those produced by all other assessments.

Abstract. Determining when and how a Loop Current eddy (LCE) in the Gulf of Mexico will finally separate is a difficult task, since several detachment re-attachment processes can occur during one of these events. Separation is usually defined based on snapshots of Eulerian fields such as sea surface height (SSH) but here we suggest that a Lagrangian view of the LCE separation process is more appropriate and objective. The basic idea is very simple: separation should be defined whenever water particles from the cyclonic side of the Loop Current move swiftly from the Yucatan Peninsula to the Florida Straits instead of penetrating into the NE Gulf of Mexico. The properties of backward-time finite time Lyapunov exponents (FTLE) computed from a numerical model of the Gulf of Mexico and Caribbean Sea are used to estimate the "skeleton" of flow and the structures involved in LCE detachment events. An Eulerian metric is defined, based on the slope of the strain direction of the instantaneous hyperbolic point of the Loop Current anticyclone that provides useful information to forecast final LCE detachments. We highlight cases in which an LCE separation metric based on SSH contours (Leben, 2005) suggests there is a separated LCE that later reattaches, whereas the slope method and FTLE structure indicate the eddy remains dynamically connected to the Loop Current during the process.

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As mesoscale oceanic eddies, they have potential long-range ability of dragging along various types of tracers (e.g., nutrients, salinity, larvae) (Robinson, 1983), and, in particular, Sargassum rafts (Beron-Vera & Miron, 2020). The argument of Huang et al. (2021) is based on the tracking of Eulerian footprints of mesoscale vortices on satellite-altimetry gridded maps of sea-surface height (SSH) anomaly (Schlax & Chelton, 2016) and absolute dynamic topography (Rio et al., 2011). Here we question Huang et al. (2021) results based on theoretical arguments, specialized SSH data analysis, and observations by Fratantoni and Richardson (2004) of satellite-tracked surface drifters and submerged floats, which show that they hardly traverse the Lesser Antilles. The main issue with the analysis by Huang et al. ( 2021) is its observer-dependent nature. We reiterate below, one more time, the issue with the Eulerian, streamline-based analysis carried out by Huang et al. (2021), no matter how sophisticated it is made (e.g., through lagged correlations and the construction of Hovmöller plots of stitched fields on one side and of the other of the Lesser Antilles) or how arbitrary it is framed (e.g., by following a particular isoline of SSH).Consider the velocity field u(x, t) = (x sin 4t + y(2 + cos 4t), x(cos 4t − 2) − y sin 4t), where 𝐴𝐴 𝐱𝐱 = (𝑥𝑥𝑥 𝑥𝑥) ∈ ℝ 2 denotes position and 𝐴𝐴 𝐴𝐴∈ ℝ is time, which represents an exact linear solution of the two-dimensional Navier-Stokes equation (Haller, 2005). The flow streamlines are closed at all times suggesting an elliptic structure, that is, a vortex. Moreover, the most common Eulerian criteria (Q, λ 2 , Δ, swirling strength, etc., cf. Haller ( 2005))